In kitchens, the grease filters and/or grease removal devices are usually installed in exhaust hoods above various types of cooking equipment including deep-fat fryers, grills, griddles and ovens where excess heat and flame can occur. In applications using existing wool or fiber-based filters (see e.g., U.S. Pat. No. 6,293,983) problems may be encountered due to excessive heat or flame. Unless stainless steel heat shields are placed in front of the wool filters, there is the possibility that concentrations of heat (e.g., from exhaust of the cooking equipment) may degrade the wool fibers and cause a hole to form in the filter. Similarly, flames may make holes in the filter if they come in direct contact with the filter which can happen above griddles and grills with a grease flare-up. This is a major draw-back as a hole in the filter renders it ineffective and requires that the filter be replaced.
When selecting the thickness of a wool filter, there is a tradeoff between airflow and grease collection. A thin filter will allow sufficient airflow but will not have a large capacity to collect and store grease. A thicker filter will allow for greater grease collection but reduce airflow. Existing filters of a thickness selected to allow sufficient airflow may not have a large capacity to catch and store grease. This renders the wool filter expensive to use as it needs to be changed very frequently, and it is also operationally inconvenient to change on a regular basis.
Additionally, existing wool filters often require a separate support frame and frame cover made of metal, which is both costly, heavy for the operator to use and susceptible to damage, often requiring costly replacement. This assembly, and often along with an optional heat shield means the current wool-based system is not only expensive to make but more significantly it can be difficult for the user to operate. As a result, operational use has been limited in the marketplace. There is room for simplification and improvement of this assembly.
The following narrative is provided to give background related to the purpose, theory and operation of kitchen exhaust hoods.
Kitchen Exhaust Hoods
Commercial and institutional cooking establishments typically employ exhaust removal systems to provide ventilation of the workspace, thus supporting the kitchen operation with the removal of heat, grease laden vapor, smoke, contaminants, and cooking by-product. The primary device utilized in the exhaust system for the purpose of capturing the plume produced by the cooking appliances, containing fire, and reducing the contaminants entering the exhaust system is the exhaust hood.
The heating of cooking appliances generally results in a natural upward draft of air, which rises from the cooking battery. At the point where food is being heated and cooked, the updraft or plume, also known as effluent, becomes entrained into the air stream rising from the cooking surface. This by-product will be produced at varying rates based on the surface temperature of the cooking appliances, and the type of foods being prepared. With cooking processes ranging from low to high temperatures, and vast differences in the grease content of the foods being prepared, the need exists to provide adequate ventilation of the workspace, contaminant removal, and fire prevention based on the severity of the cooking operation.
Non-Grease versus Grease Type Hoods
Two basic types of hoods are commonly utilized for ventilation purposes within commercial and institutional kitchen facilities, with the main factor of their division being whether they are designed to ventilate grease-laden vapor or simply heat and vapor. These are divided into type 1, (commonly referred to as class 1), constructed and designed for the venting of grease laden vapor, and type 2, (or class 2), for the venting of heat, vapor, and odor only.
Since the teachings of the present disclosure includes to the capture of grease particulates from grease laden vapor, further discussion will focus primarily upon Class 1 hoods.
Class 1 Exhaust Hoods
Cooking applications which produce grease-laden vapor typically require the use of class 1 hoods, which will be divided into two main groups; listed exhaust hoods, and unlisted hoods.
Listed exhaust hoods have been tested for performance by a third party Listing Authority, such as Intertek/ETL or Underwriters Laboratories, for their performance in capturing smoke and vapor at varying temperatures, ability to withstand fire conditions, and electrical components survivability in the application.
Grease Removal Devices
Listed and unlisted exhaust hoods, which are used in cooking applications, will include grease removal devices to reduce the grease volume of the exhaust air stream prior to the exhaust air entering the exhaust duct. Grease buildup in the exhaust ductwork is considered a fire hazard since the deposited by-product remains flammable. With the accelerated air velocities within the exhaust ductwork induced by the blower, potential risks escalate with the buildup of flammable byproduct. Failure to remove this byproduct at the source of the grease removal device within the hood may result in migration of the flammable load throughout the exhaust duct system.
Grease removal apparatus and systems are normally located within the exhaust hood, and may be either removable for cleaning, or may be a fixed component within the exhaust hood for automatic wash down, such as a water wash type hood. Removable type filters are normally arranged within a filter rack, are located just below the exhaust duct collar for the hood, and typically have a metal grease collection container, which does not typically exceed one gallon.
Baffle Filters
Most common filter hoods today utilize baffle type filters, which operate based upon the principle of centrifugal grease extraction. These filters are configured with a series of overlapping baffles, which force the grease laden exhaust air to make several changes in direction within the grease filter. The grease is dismissed from the air stream by centrifugal force, held within the filter interior, and accumulation of the grease aerosol particles which begin to liquefy continuously drain from the filter to a grease drip tray, or trough, and then drain into a metal container which does not typically exceed one gallon.
Baffle filters operate with among the lowest resistance offered by any grease removal device, which may be between 0.50″ and 0.75″ static pressure, and the average air velocity at the face of the filter may be between 150 feet per minute, and 400 feet per minute, both depending upon the hood airflow in volume.
Baffle filters are listed to Underwriters Laboratories (“UL”) Standard 1046, which tests their ability to perform in the application, and in fire conditions. However, UL Standard 1046 does not adequately address the effectiveness of the baffle to actually remove grease from the air flow and prevent it from entering the exhaust duct. As a result, most baffles are very poor at removing grease from the airflow.
Grease in the ducting constitutes a major fire hazard as duct fires are a constant concern in commercial kitchen exhaust systems.